Abstract: Lactobacillus plantarum K25, isolated from Tibetan Kefir, was pretreated at low (4, 10 or 20 ℃) and high temperatures (45, 50 or 55 ℃) for different times (0.5, 1.5, 3, 5, 8, 18 or 24 h). Subsequently, the survivability of L. plantarum K25 was determined after 1, 15 and 30 days of storage at ?20 ℃. L. plantarum K25 without any treatment was used as a control. The results showed that after pretreatments at 4 ℃ for 18 h and at 55 ℃ for 0.5 h the viable counts were 12 and 15 times higher than those of the control after 30 days of storage at ?20 ℃. Fermented ice cream using K25 (control), C-K25 (pretreated at 4 ℃ for 18 h) and H-K25 (pretreated at 55 ℃ for 0.5 h) with or without chocolate were prepared. The acidity of the C-K25 and H-K25 fermented ice cream was lower than that of the control, whereas the viscosity and firmness were significantly increased as compared to those of the control (P < 0.05). In addition, melting rate and overrun were improved in the C-K25 and H-K25 fermented ice cream. The viable counts of the control, H-K25 and C-K25 ice cream were 5.63, 7.26 and 7.32 (lg(CFU/mL)), respectively after 60 days of storage at ?20 ℃, indicating that cold and heat pretreatments facilitated bacterial survival in the frozen environment. The highest viable count (7.56 (lg(CFU/mL))) was found in the C-K25-fermented ice cream with chocolate, indicating that chocolate provided protection for L. plantarum K25 against cold damage. Finally, after storage at ?20 ℃ for 30 days, all the experimental groups showed no significant decrease in the survival rate of L. plantarum K25 upon mimic gastric acid and bile salt stress (P > 0.05).

Key words: ice cream, temperature stress treatment, viable count, mimic gastrointestinal environment